Only off when mad dogs go out? (4)

I believe the answer is:

noon

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'when mad dogs go out?' is the definition.
('mad dogs and go out in the midday sun')

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for noon that I've seen before include "[LUNCHTIME] ?" , "Twelve during daylight" , "See 8" , "12 o'clock" , "when only mad dogs go out?" .)

I've seen this clue in The Guardian.

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